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Thread Title:
This seems to imply the desire for a formula.
Following is the pertinent information provided by jkpyke:
Post 1:
I can't seem to find a formula for a practice question that I have. I am supplied the wattage, the H.P. and the voltage for the 3-phase motor, and it is asking me what the current is. Can anyone let me know what the formula would be for this?
At this point there is no information that the practice question is a multiple N-choice question. Since the request for a formula is the question of this post the only answer is that there is no formula. There can not not be a specific solution to the practice question.
Post 7:
Thanks for the help guys. Sorry Gar, there is an answer for this, it was actually on my State test and that was all the info given in the question and I did not know how to answer it/calculate it.
This has now changed to a State Test and not a practice question. Also now the problem is "I did not know how to answer it". Good reason, there is no solution.
Post 14:
--- Anyways, the question was "A 3-phase motor drawing XXXX watts, that is XX H.P. and XXX volts, calculate the current." Then it said "Hint not FLC" The question was multiple choice. Sorry I did not want to come out and say that it was from a state test from the beginning and I don't remember the exact numbers of the questions, since I took the test 2 weeks ago. This question has been stuck in my head since the test. It is not the fact of whether or not that I got the question right or wrong, but on how to calculate it correctly that has been bothering me. Gar is correct in his response, and since I am from the same state that Gar is from, he should know how the Journeyman test is. I can't really argue with the people from the State giving me my Journeyman test. Either way it has made me really think.
Now we learn this is a multiple choice type of question. A whole different set of conditions. Now it is a guessing game. I have no idea what is the purpose of the hint, but the numbers are not available. Maybe the HP given was the HP rating of the motor, and by the power input it is clear that the motor is running at partial load.
Suppose an entire test is made up of questions with four possible answers provided, and the requirement is that you must choose one of the given possible answers. With no knowledge of the subject whatsoever, and you randomly pick answers your score on average will be 25%. If the desired answers were randomly positioned, then you could pick the first position for all answers and on average get a 25% score.
Now continue with a person with no knowledge of the subject, but a good understanding of the language, English in this case, and just by the structure of the question and the answer choices and intelligent guessing can probably do much better than 25%.
Now to the specific question here. We have no numbers with which to work. So it is a bigger guessing game for us.
Suppose that, as some have suggested, that watts was the power input, and HP was the power output. I will pick two values from the motor performance table I presented in an earlier post. 17,000 watts and 19.5 HP. So convert HP to watts and the result is 14,547. This seem reasonable because it is 86% of the input power. We do not know the motor rating, but we guess at 90% for the power factor assuming the motor is operating near its rating. With input power, assumed power factor, known voltage (440 here), and that the motor is a balanced load we can estimate line current. Theoretical line to neutral voltage is 440/1.732 = 254 V. Input VA per phase is 17,000/(3*0.9) = 6296 VA. Thus, I = 6296/254 = 24.8 A. The actual value from the table is 25 A which I had ask the reader to determine.
Now is 24.8 A sufficiently close to any one of the multiple answers provided. At this point you make a judgement. Suppose you are midway between two answers, then you need to rethink your assumptions and make a judgement of which way to go for your answer choice.
Tests with multiple choice answers make it easy for grading the tests, but may not do a good job of teaching and determining a student's real understanding of the subject.
To answer the question you are really dealing with the need for basic knowledge of motors, some equations, some practical data, and common sense to answer this type of question. How ridiculous the non-desired answers are determines how easy it is to answer the question.
Suppose some answers for my 35 HP motor were 19.1, 22.3, 25.0, 34,3, 43.5 A what would you pick given 35 HP, 17 KW, and 440 V delta were the input values of the question?
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